UUID v4 Decoder and v7 Checker: Reading Version and Timestamp

Version and variant

The first character of the third group is the version. The first character of the fourth group is the variant, and for RFC 9562 UUIDs it is 8, 9, a or b:

36b8f84d-df4e-4d49-b662-bcde71a8764f   -> version 4 (random)
0191a2b3-c4d5-7abc-8def-0123456789ab   -> version 7 (time-ordered)
6ba7b810-9dad-11d1-80b4-00c04fd430c8   -> version 1 (time + MAC)

A value whose variant nibble is outside 8-b is not an RFC 9562 UUID — usually a hand-rolled generator that filled all 128 bits randomly.

Extracting a v7 timestamp

The first 48 bits of a v7 are Unix milliseconds, so creation time reads straight out:

const ms = parseInt(uuid.replace(/-/g, '').slice(0, 12), 16);
new Date(ms).toISOString();

v1 is different: its timestamp counts 100-nanosecond intervals since 1582 and is split across three fields in reverse order, so it needs reassembling rather than a simple slice.

What a v4 tells you

Nothing beyond version and variant. There is no timestamp, no machine identifier and no sequence — 122 random bits and no structure to recover.

That is exactly why v4 remains the right choice for public identifiers: a decoder learns nothing about when the record was created or how many exist.

Validating

^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[1-8][0-9a-fA-F]{3}-[89abAB][0-9a-fA-F]{3}-[0-9a-fA-F]{12}$

This accepts any RFC version while still checking the variant. Pin the version digit if you require a specific one — and normalise case before comparing, since hex is case-insensitive and RFC 9562 asks for lowercase output.